The present invention relates to an intake system for an internal combustion engine such as a diesel engine or a gasoline engine.
An EGR (Exhaust Gas Recirculation) device is known in which a part of exhaust gas of an internal combustion engine such as diesel engine or gasoline engine is brought into an intake air and thereby a quantity of air which flows into the engine is reduced to lower a combustion temperature, in order to reduce a concentration of NOx within the exhaust gas and in order to improve a fuel economy. Moreover, a structure is known in which a turbocharger using exhaust gas is provided to the internal combustion engine and there are provided a low-pressure EGR passage for bringing a part of exhaust gas from a portion of exhaust passage located downstream beyond a turbine of the turbocharger back to the intake air, and a high-pressure EGR passage for bringing a part of exhaust gas from a portion of the exhaust passage located upstream beyond the turbine back to the intake air.
On the other hand, in the EGR device, a flow quantity (flow rate) of EGR gas (hereinafter, also referred to as “exhaust gas”) included in a mixture gas of fresh air and exhaust gas needs to be adjusted by disposing various sensors at intake and exhaust passages and by monitoring a mixing state of the mixture gas by means of these sensors. Therefore, Japanese Patent Application Publication No. 2008-261300 (see paragraph [0014]) discloses a previously proposed EGR device. In this technique, for an engine having a turbocharger, an oxygen sensor is disposed downstream beyond a connecting (pipe-junction) portion between a low-pressure EGR passage and an intake passage, and thereby, flow quantities of low-pressure EGR passage and high-pressure EGR passage are controlled according to a concentration of CO2 included in a mixture gas flowing within the intake passage. The document of Japanese Patent Application Publication No. 2008-261300 says that the fresh air is sufficiently mixed with the low-pressure EGR gas at a downstream location beyond the junction portion so as to form a mixture gas having a constant pressure, and that the CO2 concentration included in this constant-pressure mixture gas can be accurately measured.